1. Field of the Invention
This invention relates to a surface acoustic wave device and a production process thereof.
Generally, a surface acoustic wave (SAW) device comprises a piezoelectric substrate and a comb-shaped interdigital electrode disposed on the substrate, for converting a voltage to a surface acoustic wave or vice versa. The function of the surface acoustic wave device is to convert a radio frequency voltage to a surface acoustic wave having a wavelength of about 10xe2x88x925 times by using a comb-shaped interdigital electrode, which causes this wave to propagate on the surface of the piezoelectric substrate and converts again the wave to the voltage by the comb-shaped interdigital electrode.
Frequency selectivity can be provided in accordance with the shape of the interdigital electrode during the two conversion operations between the surface acoustic wave and the voltage, and a filter or a resonator can be constituted by utilizing this characteristic property. Because the propagation speed can be retarded to about 10xe2x88x925 times that of an electromagnetic wave, the surface acoustic wave device can be used as a delay device.
The application of the surface acoustic wave device to small, economical filters, resonators, delay lines, etc., has already been done by utilizing the functions described above. In other words, the surface acoustic wave device has been applied to IF filters of television sets, resonators of VTR (vide tape recorder) oscillators, VCOs of cordless telephones, and recently, the application has been expanded to RF filters and IF filters of automobile telephones, mobile telephones, and so forth.
To further expand the utilization in this field, it is important to improve a pass band and power characteristics of the surface acoustic wave device. Particularly in the case of the automobile telephones and the mobile telephones, transmission power is relatively great, that is, 0.6 to 3 W, and a large RF power is applied to a filter of a front-end portion inside the apparatus, particularly, to an antenna duplexer.
The maximum input power of the surface acoustic wave filter has been about 0.2 W up to the present, and the filter lacks sufficient power characteristics. For this reason, a dielectric filter having high power resistance has been used for the antenna duplexer. However, because the dielectric filter is large in scale, it causes a problem when the size of the apparatus is reduced as a whole.
Accordingly, if the power characteristics of the surface acoustic wave device can be improved and the antenna duplexer can be realized by utilizing the surface acoustic wave device, the mobile telephones can be made even smaller, and the effect of utilization in industry becomes greater.
2. Description of the Related Art
The interdigital electrode is used in the surface acoustic wave device as described above, and aluminum (Al) or an aluminum alloy containing a small amount of a different kind of metal (not always a solid solution body in many cases) is generally used because the mass is small and its electrical resistance value is low.
Several proposals have been made for the structure of the antenna duplexer using the surface acoustic wave device. Typical examples are described in Japanese Unexamined Patent Publication (Kokai) Nos. 5-167388 and 5-167389. In order to simplify the filter structure in the duplexer and to secure desired characteristics, Japanese Unexamined Patent Publication (Kokai) No. 5-167388 proposes to constitute a duplexer by using a plurality of band-pass filters each formed by using the surface acoustic wave device. Japanese Unexamined Patent Publication (Kokai) No. 5-167389 proposes to integrate a plurality of surface acoustic wave band-pass filter chips having mutually different center frequency bands and having signal input/output terminals and ground terminals, by storing them in one package so as to minimize the duplexer while keeping excellent isolation.
However, the conventional antenna duplexer does not have characteristics such that the filter can sufficiently withstand the increase of RF power. To evaluate the power resistance or characteristics, the life time at the maximum input power at which the apparatus can be used is generally used as a guideline. The conventional antenna duplexer has a life time of only about 1,600 hours at the 1 W input at an environmental temperature of 85xc2x0 C. (chip temperature of 120xc2x0 C.) in an accelerated deterioration test stipulated for the mobile telephones of the NTT specification in Japan, for example. These values are not considered sufficient for the life of mobile telephones, and the values of at least twice are believed necessary.
The main factor that determines the useful life of the surface acoustic wave device is power characteristics of electrode fingers of the filter (interdigital electrode fingers IDT), and an aluminum system alloy film containing a trace amount of copper and formed by sputtering, which is well known as being resistant to migration in the field of semiconductor devices, has been used. However, this alloy is not yet sufficient as the electrode material of the surface acoustic wave device used as the antenna duplexer to which a high power load is applied.
Besides the patent references described above, the following reports have been made regarding the methods of improving electric power of the electrode of the surface acoustic wave device.
1. Change of addition metal in aluminum (Al) system alloy:
The use of an aluminum-titanium alloy (Al-Ti), etc., for example, is described in detail in xe2x80x9cExamination of Al System Thin Film Material for SAW Power-Resistant Electrode and Production Method Thereofxe2x80x9d (by Yuhara et al.), No. 17th EM Symposium Presume, pp. 7-12. According to this report, the useful life of the surface acoustic wave device can be improved by about 10 times the life of an aluminum-copper (Al-Cu) alloy film by changing the electrode material to an aluminum-titanium (Al-Ti) alloy.
2. Use of aluminum (Al) epitaxial single crystal film:
This method is based on the fact that grain boundary diffusion in stress migration of aluminum (Al) can be restricted by converting the structure to a single crystal, and is reported in papers of the Electronic Data Communication Society, A, Vol. J76-A, No. 2, pp. 145-152 (1993) (by Ieki et al.). According to this report, life time can be improved to 2,000 times that of an aluminum-copper alloy (Al-Cu) film by vacuum evaporation.
In comparison with films formed by sputtering, the useful life of an aluminum-copper alloy (Al-Cu) film formed by vacuum evaporation is much shorter from the beginning (refer to Yuhara et al., and other references), and the improvement in life time is believed to be substantially 20 to 200 times. At present, however, it has been confirmed only that this method can cause epitaxial growth only when the substrate material as the base is quartz, and cannot realize the film when LiTaO3 or LiNbO3, which have been widely used as a substrate material for filters for mobile communication, is employed.
As described above, stress migration in the surface acoustic wave device is analogous to electromigration and stress migration in wiring technology of semiconductor devices, and migration-resistant technology in the semiconductor devices will be useful for the migration-resistant technology in the surface acoustic wave devices. Among them, the following technology has drawn increasing attention.
Namely, it is the method which forms in a laminar form a film of an intermetallic compound of aluminum (Al) and a transition metal between the aluminum (Al) films so as to block electromigration of the aluminum (Al) atoms by the intermetallic compound. This method is reported in U.S. Pat. No. 4,017,890 (J. K. Howard, IBM, April 1977) and in connection with this patent, a report is made by J. K. Howard, J. F. White and P. S. Ho in xe2x80x9cJ. Appl. Phys., Vol. 49, p. 4083 (1978).
According to these reports, life time becomes maximal when chromium (Cr) is used as the transition metal, and is about 10 times that of the aluminum-copper alloy (Al-Cu). However, when the inventors of the present invention applied this method to the electrode of the surface acoustic wave device, a sufficient effect could not be obtained.
As described above, several methods have been proposed as the prior art technologies for improving the electrode materials, but none of them have provided sufficient power characteristics. Accordingly, development of an electrode material having higher performance has been necessary. As a matter of fact, when the method of improving the power characteristics by the multi-layered structure of the aluminum films (Al) and the intermetallic compound of the aluminum (Al) and the transition metal is applied to the surface acoustic wave device, no effect can be observed but performance actually deteriorates.
FIG. 1 is an explanatory structural view of a surface acoustic wave filer having the conventional three-layered structure. In the drawing, reference numeral 11 denotes a LiTaO3 piezoelectric substrate, 12 is an Al-1%Cu alloy film, 13 is a Ta film, 14 is an Al-1%Cu film, and 15 and 16 are Al-Ta alloy films.
In the surface acoustic wave filter using this conventional three-layered electrode structure, an 1,000 xc3x85-thick Al-1%Cu alloy film 12 is formed on the LiTaO3 piezoelectric substrate 11, a 500 xc3x85-thick Ta film 13 is formed on the former, and a 1,000 xc3x85-thick Al-1%Cu film 14 is further formed on the Ta film 13. Next, heat-treatment is carried out at 400xc2x0 C. in vacuum so as to form sufficient Ta-Al (TaAl3) 15 and 16 on the interface between the Al-1%Cu films 12, 14, and the Ta film 13 and in the grain boundaries of the Al-1%Cu alloy films 12, 14. The electrode structure is then patterned into an interdigital shape to form the electrode. When the useful life of this surface acoustic wave filter is measured by conducting an accelerated deterioration test at a chip temperature of 120xc2x0 C. and radio frequency power of 1 W, the life expectancy is found to be 100 hours, and drops to {fraction (1/16)} of the life time of a 3,200 xc3x85-thick Al-1%Cu single layered film, that is, 1,600 hours.
FIG. 2 is a graph useful for explaining power characteristics of a surface acoustic wave filter having the conventional three-layered structure.
In the graph, the abscissa represents input power (W) and the ordinate represents life time (mean time to failure: MTTF, hours). Curve a represents an Al-1%Cu single layer film which is not heat-treated, curve b represents an Al-1%Cu/Ta/Al-1%Cu film which is not heat-treated, and curve c represents an Al-1%Cu/Ta/Al-1%Cu film which is heat-treated at 400xc2x0 C. The substrate (chip) temperature when forming each film is 120xc2x0 C., and each film has a thickness of 3,200 xc3x85.
According to the J. K. Howard et al. reference described above, the surface acoustic wave filter having the three-layered structure electrode described above should provide longer life at least 20 times that of the Al-1%Cu film. According to experiments, however, the actual life of the Al-1%Cu/Ta/Al-1%Cu film (see curve c) which is formed under the ordinary heat-treatment conditions at 400xc2x0 C. is much shorter than the life of the Al-1%Cu single layer film (see curve a) which is not heat-treated. This difference results from some differences of a life deterioration mechanism of wirings of semiconductor devices from a life deterioration mechanism of IDT (Interdigital Transducer) of the surface acoustic wave filter.
In short, both electromigration of the Al atoms and static stress migration are involved in the life deterioration of the wirings of the semiconductor devices, whereas the life deterioration of IDT of the surface acoustic wave device mainly results from the dynamic stress migration. Here, the static stress migration means the Al migration driven by the static internal stress of Al films. The dynamic stress migration means the Al migration driven by the dynamic migration of the internal stress caused by the acoustic surface wave propagation. Depending on parameters associated with the life deterioration, exactly opposite actions results in some cases due to the difference of electromigration from the dynamic stress migration. A typical example is the grain size of Al. According to J. B. Ghate, xe2x80x9cElectro-migration-Induced failure VLSI Interconnectorsxe2x80x9d, Solid State Technology, pp. 113-120, 1983, the greater the grain size, the greater the effect of suppressing electromigration and the longer life becomes, in the case of the wirings of the semiconductor devices. On the other hand, according to the aforementioned Yuhara et al. reference, the greater the grain size, the shorter life becomes, in the case of the surface acoustic wave device.
FIG. 3 is a graph useful for schematically explaining the relation between the grain size of the electrode material and life time. The abscissa in the graph represents the grain size, and the ordinate represents life time. As shown in the graph, since electromigration is predominant in the case of the wirings of the semiconductor device, life time becomes longer with the increase of the grain size (see curve b). In the case of the surface acoustic wave (SAW) device electrode, on the other hand, since stress migration is predominant, life time becomes shorter with the increase of the grain size of the electrode material (see curve a). The grain size of the electrode material can be increased by applying heat-treatment.
It can be interpreted from the sequence described above that the cause of deterioration of the Al-1%Cu/Ta/Al-1%Cu film formed conventionally by applying heat-treatment at 400xc2x0 C. and represented by the curve c in FIG. 2 is this heat-treatment at 400xc2x0 C., since the grain size becomes greater and stress migration becomes more likely to occur due to this heat-treatment, so life time is reduced. To further support this fact, a three-layered film having the same structure is formed without carrying out the heat-treatment and moreover, in such a manner that the temperature never exceeds 200xc2x0 C. throughout the full process, so as to constitute the surface acoustic wave filter. When life of this filter is evaluated, the curve b in FIG. 2 can be obtained, and life time is substantially equal to that of the Al-1%Cu single layered film (see curve a).
It can be understood that when the heat-treatment is not carried out at a high temperature of about 400xc2x0 C., life time can be drastically improved. This is because the grain size can be kept small. In this case, although the grain size remains small and life time is relatively long, the alloy between Al and the transition metal is not formed, between the layers because heat-treatment is not effected, and because the function of a stopper for inhibiting cracks occurring in the film, that is, the growth of voids, does not exist, so life time is not improved in comparison with the Al-1%Cu single layered film which is not heat-treated (see curve a).
It is an object of the present invention to provide a surface acoustic wave device which can prevent the occurrence of voids in a film while keeping a grain size of an Al-Cu multi-layered film small, and which has a long life time.
It is another object of the present invention to provide a process for producing such a surface acoustic wave device.
These and other objects of the present invention will become more apparent from the following detailed description of preferred embodiments thereof.
According to the present invention, there is provided a surface acoustic wave device which comprises a piezoelectric substrate and an electrode formed on the substrate by alternately laminating a film of aluminum containing at least copper added thereto or an alloy of such aluminum and a copper film. In this case, the electrode is a transducer for converting an electrical signal to a surface acoustic wave.
In the surface acoustic wave device according to the present invention, directions of internal stresses of the film of aluminum containing at least copper or the alloy of such aluminum and the copper film preferably have opposite directions, and moreover, the sum of these internal stresses are zero (0) or compressive (stress on the negative side). When the internal stresses are regulated in this way, stress migration of aluminum can be reduced.
A laminate structure of the aluminum or aluminum alloy film/copper film constituting the electrode can be constituted arbitrarily into a two- or more multi-layered structure, and is preferably a two- or three-layered laminate structure. In such a multi-layered structure, the thickness of each film can be broadly changed in accordance with frequency and other various factors, but is generally and preferably within the range of from about 300 xc3x85 to about 10,000 xc3x85.
In a preferred embodiment of the present invention, the electrode can be a two-layered laminate structure of the aluminum-copper alloy film and the copper film. Here, the thickness of the Al-Cu film for 800 to 1,000 MHz filters is preferably from about 1,000 xc3x85 to about 5,000 xc3x85, and the thickness of the Cu film is preferably from about 300 to about 1,000 xc3x85.
In another preferred embodiment of the present invention, the electrode can be a three-layered laminate structure comprising two aluminum-copper alloy films and the copper film sandwiched between the aluminum-copper alloy films. The thickness of each of the Al-Cu films for 800 to 1,000 MHz filters is preferably from about 500 xc3x85 to about 1,500 xc3x85, and the thickness of the Cu film is preferably from about 300 xc3x85 to about 1,000 xc3x85.
In the surface acoustic wave device according to the present invention, it is essentially necessary to add copper to the aluminum or aluminum alloy film constituting the electrode. The amount of addition of copper is preferably from 0.4 to 4 wt% and further preferably, from 0.5 to 1.5 wt% on the basis of the weight of the film. If the amount of addition of copper is below 0.4 wt%, problems such as stress migrations appear, and if it exceeds 4 wt%, on the other hand, fine patterns of IDT can not be delineated by RIE (reactive ion etching) because of copper-based residue.
In the embodiments of the present invention, copper is most preferably added to the aluminum or aluminum alloy film.
The piezoelectric substrate used as the substrate can be those piezoelectric crystal substrates which are ordinarily used in surface acoustic wave devices, such as LiNbO3, LiTaO3, quartz, ZnO/glass, PZT type ceramics, and so forth. Preferably, LiTaO3, such as (36xc2x0 Y-X)LiTaO3 and LiNbO3 such as (64xc2x0 Y-X)LiNbO3, can be used effectively as the piezoelectric substrate.
When a surface acoustic wave device having a piezoelectric substrate and an electrode formed on the substrate is produced, the present invention provides a process for producing a surface acoustic wave device which comprises alternately laminating a film of aluminum containing at least copper added thereto and a copper film on the piezoelectric substrate at a temperature not higher than 200xc2x0 C.; patterning the resulting laminate structure to form an electrode; and carrying out subsequent processings while maintaining the temperature of not higher than 200xc2x0 C.
The piezoelectric substrate and the electrode formed on the substrate have already been described above. The electrode can be formed by laminating the respective films into a predetermined film thickness by ordinary film formation technology such as sputtering, CVD (Chemical Vapor Deposition), electron beam deposition, etc., and subsequently patterning the resulting laminate structure into a desired electrode shape.
The method of the present invention can restrict the growth of the grain boundary of the electrode materials by employing the process steps described above.